Nitric oxide synthase (NOS) is a heme-dependent monoxygenase that catalyzes the conversion of L-arginine to L-citrulline, producing nitric oxide (NO), through a five electron process involving oxidative removal of the imine nitrogen of the guanidino moiety of endogenous L-arginine. The active form of the enzyme is a homodimer, which requires flavin, heme, NADPH, and tetrahydrobiopterin prosthetic groups. (See, Moore, P. K.; Handy, R. L. C. Selective Inhibitors of Neuronal Nitric Oxide Synthase-Is No NOS Really Good NOS for the Nervous System? Trends in Pharmaceutical Science 1997, 18, 204-211.) Three isoforms of NOS are known: a form found in the brain and mitochondria (nNOS), a form found in the endothelium (eNOS), and an inducible form expressed during immune system response (iNOS). All three isoforms use the same substrate. Under normal circumstances, NO is an important second messenger, activating the formation of cyclic guanosine 3′,5′-monophosphate (cGMP).
Oxidative stress caused by excess NO has been implicated in the pathogenesis of certain neurodegenerative diseases, including cerebral palsy and Parkinson's disease (PD). In both diseases, high levels of NO are implicated in the degeneration of neurological tissues. In the case of cerebral palsy, a mechanical complication during the birthing process can lead to low oxygen levels in the fetal brain—causing cell membrane depolarization due to energy starvation, which in turn floods the cells with calcium. NOS is also activated—producing excess NO, which is toxic for mitochondria and leads to apoptosis. Excess NO is linked to the progression of PD on several fronts, such as protein aggregation due to nitrative modification and misfolding, disruption of systems involved in degradation and clearance of aberrant proteins, as well as inactivation of regulatory mechanisms of cell death pathways.
It is believed that selective inhibition of nNOS under pathological conditions could be neuroprotective. (Ji, H.; Tan, S.; Igarashi, J.; Li, H.; Derrick, M.; Martasek, P.; Roman, L. J.; Vasquez-Vivar, J.; Poulos, T. L.; Silverman, R. B., Selective Neuronal Nitric Oxide Synthase Inhibitors and the prevention of Cerebral Palsy, Ann. Neurol. 2008, 64, 1-9.) It is also understood that eNOS is involved in vasoconstriction, which mediates blood pressure. The structures of both isoforms are very similar. Therefore, any therapeutic benefit resulting from non-selective NOS inhibition is counter balanced by a dangerous increase in blood pressure. Accordingly, the search for a selective inhibitor of nNOS has been ongoing in the art.